This application is based on Application No. 2000-383463, filed in Japan on Dec. 18, 2000, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a battery charger provided with a cell shunt (charging current shunt) circuit capable of protecting the overcharge of a plurality of individual lithium ion battery cells connected in series with one another to constitute a lithium ion battery for example, which can be installed on a satellite, spacecraft, etc.
2. Description of the Related Art
FIG. 6 illustrates a known lithium ion battery charger. The known battery charger includes a plurality of lithium ion battery cells C1-Cn, and a plurality of cell shunt circuit sections Sh1-Shn, and a constant current controller 8. Each of the cell shunt circuit sections Sh1-Shn is constituted by a shunt transistor 4, a reference voltage generating section 5, and a comparator 7. In addition, a symbol Ichg represents a battery charging current, IP a shunt current, Vc a cell voltage, and Vs a reference voltage generated by the reference voltage generating section 5.
In a nickel cadmium battery and a nickel hydrogen battery used in the past as batteries for a satellite and a spacecraft, a plurality of battery cells connected in series with one another are charged by performing batch constant-current charging in order to achieve reduction in resources of a battery charger and improvements in reliability. On the other hand, lithium ion battery cells, which have high energy density and various excellent characteristics for installation on a satellite and a spacecraft, are becoming the mainstream of future satellite and spacecraft batteries.
In such a batch constant-current charging method, however, variations in individual capacities of the battery cells result in variations in the charging amounts for the respective cells, so that some battery cells may be overcharged.
In particular, lithium ion battery cells have a characteristic that the life-time property is remarkably decreased due to overcharging. This is a weak point for satellite and spacecraft batteries for which a long life time is required. Thus, the lithium ion battery charger as referred to above is required in order to cope with such a problem.
The known lithium ion battery charger is constructed as shown in FIG. 6. In this known battery charger, the constant current controller 8 performs power conversion based on the electric power from a primary power supply input section 10 to generate, through constant current control, a battery charging current Ichg, which is supplied to the lithium ion battery cells C1-Cn connected in series with the constant current controller 8 to charge the lithium ion battery cells C1-Cn in a batch processing manner.
By this charging, charge energy is uniformly accumulated in the respective lithium ion battery cells C1-Cn in proportion to the product of the battery charging current Ichg. and a charging time or duration It. In this process, a cell voltage Vc in each of the lithium ion battery cells C1-Cn rises according to a charge energy limit of each cell or variations in the capacities of the cells. Although a cell having the smallest capacity first reaches a charging completion voltage, charging is continued until all the lithium ion battery cells C1-Cn reach the charging completion voltage.
In this charging operation, when a lithium ion battery cell C1 first reaches the charging completion voltage for instance, the comparator 7 in the cell shunt circuit section Sh1 detects that a voltage Vc across the cell C1 reaches a reference voltage Vs which is preset to a value equal to the charging completion voltage, and drives the shunt transistor 4, so that a surplus current (hereinafter, called a shunt current Ip) is shunted from the battery charging current Ichg, thus preventing the battery charging current Ichg from being supplied to the lithium ion battery cell C1. The above-mentioned operation is similarly performed in each of the cell shunt circuit sections Sh1-Shn, whereby each of the lithium ion battery cells C1-Cn is prevented from being overcharged in the continued charging operation.
In the lithium ion battery charger as described above, by shunting the shunt current Ip from the battery charging current Ichg through each shunt transistor 4, the lithium ion battery cells C1-Cn are prevented from being overcharged. However, the shunt current Ip flows through each shunt transistor 4 to generate heat P therein, as expressed by the following equation (1).
P=Ichgxc3x97Vcxe2x80x83xe2x80x83(1)
This heat P makes the thermal design of a satellite or spacecraft battery system difficult. Moreover, it is a factor of disturbing the reduction in resources and the improvement in reliability of the battery charger.
The present invention is intended to solve above-mentioned drawbacks, and the object of the present invention is to suppress the generation of heat in a battery charger.
Bearing the above object in mind, the present invention resided in a battery charger for charging a plurality of battery cells connected in series with one another with a constant current in a batch manner, the battery charger including a plurality of cell shunt circuit sections connected in parallel with the battery cells, respectively, a plurality of cell voltage detecting sections for detecting cell voltages of the battery cells, respectively, and a cell shunt driving unit for driving the cell shunt circuit sections, respectively. Each of the cell shunt circuit sections comprises: an energy reservoir acting as a bypass path for bypassing a charging current supplied to a corresponding one of the battery cells so as to be input to the following battery cell provided at a downstream side of the one battery cell so as to reserve surplus energy obtained from the charging current thus bypassed and regenerate the thus reserved surplus energy to a batch charging line connected with the serially connected battery cells; and a switching element inserted in the bypass path for opening and closing thereof. The cell shunt driving unit compares a voltage detected by each of the cell voltage detecting section with a reference voltage, and outputs to the switching element of each of the cell shunt circuit sections a drive pulse signal of a pulse width corresponding to a difference between the detected voltage and the reference voltage. The energy reservoir regenerates the surplus energy to the batch charging line when the switching element opens the bypass path based on the drive pulse signal from the cell shunt driving unit.
In a preferred form of the present invention, the energy reservoir comprises a flyback transformer having a primary winding and a secondary winding, the flyback transformer being operable to bypass a battery cell charging current flowing in its primary winding through the switching element and regenerate an output of its secondary winding in a direction of the batch charging line through a reverse-flow preventive diode.
In another preferred form of the present invention, the cell shunt driving unit comprises a plurality of cell shunt driving sections provided one for each of the cell shunt circuit sections. Each of the cell shunt driving sections comprises: a differential amplifying section for comparing the voltage detected by a corresponding one of the cell voltage detecting sections and the reference voltage to output a difference therebetween; and a switch driving section for controlling the pulse width of the drive pulse signal in accordance with an output of the differential amplifying section thereby to control the opening and closing of a corresponding one of the switching elements based on the thus controlled pulse Width.
In a further preferred form of the present invention, the cell shunt driving unit comprises a plurality of cell shunt driving sections provided one for each of the cell shunt circuit sections. Each of the cell shunt driving sections comprises: a comparator for comparing the voltage detected by a corresponding one of the cell voltage detecting sections with the reference voltage to generate an output signal representative of a comparison result; and a fixed pulse width switch driving section for outputting a fixed driving pulse signal with a preset pulse width to the corresponding switching element based on the output signal of the comparator.
In a yet further preferred form of the present invention, the cell shunt driving unit comprises a cell shunt driving section provided in common for the plurality of cell shunt circuit sections. The cell shunt driving section comprises: a switching section including a plurality of switches connected with the switching element s, respectively; a switch selection controller for selecting one of the switches of the switching section to control the opening and closing thereof; a multiplexer controller for performing a selection setting of the voltage detected by each of the cell voltage detecting sections and notifying the selection setting to the switch selection controller; a multiplexer for successively selecting and outputting one of the voltages detected by the plurality of cell voltage detecting sections based on the selection setting of the multiplexer controller; a comparator for comparing the detected voltage output from the multiplexer with the reference voltage to generate an output signal representative of a comparison result; and a fixed pulse width switch driving section for outputting a fixed driving pulse signal with a preset pulse width. The switch selection controller closes, when detecting based on the output signal of the comparator that the detected voltage is greater than the reference voltage, the switch selected based on the selection setting of the multiplexer controller, and outputs the fixed driving pulse signal from the fixed pulse width switch driving section to a corresponding one of the switching elements through the closed switch.
The above and other objects, features and advantages of the present invention will become more readily apparent to those skilled in the art from the following detailed description of preferred embodiments of the present invention taken in conjunction with the accompanying drawings.